564 INTERRELATIONSHIPS OF ORGANISMS 



changed agricultural practices and the decrease of the horse population 

 in towns and cities; and partly (once all suitable physical environments 

 had been occupied) to the great competition for food and nesting sites 

 that developed within the sparrow population. 



From 1916 through 1920, the U.S. Biological Survey carried on a 

 detailed bird census. They found that in the North Central states there 

 was an average English sparrow population of 11 nesting pairs to 100 

 acres; and within this 5-year period the population fluctuated between 

 9 and 13 pairs per 100 acres. According to the estimate made in 1889, each 

 100 acres should have produced 575 sparrows each year, but in the period 

 studied they were just holding their own. In the Northeastern states the 

 environmental resistance was greater, for there the nesting pairs aver- 

 aged but 5, with a fluctuation between 3 and 7 pairs per 100 acres during 

 this same 5-year period. 



Numerous examples are on record of some imported insect that, freed 

 from its native parasites and predators, in the presence of an abundant 

 food supply, increased at a rate that seemed comparable to the calculated 

 biotic potential, but when its native enemies had been imported, became 

 reduced to a small, stable population. 



Population Fluctuations. Another illustration of the relationship be- 

 tween biotic potential and environmental resistance is found in the 

 sporadic outbursts of tremendous numbers of certain animals from their 

 normal habitats and geographic boundaries, to occupy and often to 

 become great pests in adjoining territories. Plagues of rats, field mice, and 

 locusts that arrive in countless hordes, eat the country bare of standing 

 crops, and then move on or soon die off are well known. They illustrate 

 the tremendous potential reproductive power that can quickly produce a 

 thousandfold increase in a normal population whenever the environ- 

 mental resistance is reduced by a few seasons of unusually favorable 

 conditions. On a small local scale such rapid fluctuations in the numbers 

 of various (often nonpest) species are common and may be seen in most 

 years in nearly any region. 



Some Biotic Factors of the Environmental Resistance 



We have already seen, at least by implication, that a part of the en- 

 vironmental resistance is due to the physical environment. Any departure 

 from the optimum conditions of temperature, humidity, light, etc., 

 operates to increase the time required to produce another generation and 

 to decrease the number of offspring. We have also referred to the com- 

 petition for food and nesting sites and the mortality that is caused by 

 parasites and predators. These last are examples of biotic factors of 

 environmental resistance that are discussed in more detail below. 



